1. Field of the Invention
The present invention relates to concentrated liquid cleansing compositions suitable for topical application for cleansing the human body, such as the skin and hair. In particular, it relates to a concentrated, lamellar phase personal cleansing composition that is able to lather appreciably and in a preferred embodiment resist freeze-thaw destabilization.
2. Background of the Art
Concentrated lamellar phase liquid cleansers are known, For example U.S. Pat. Nos. 7,884,060 and 7,884,061 issued on Feb. 14, 2012 and 8,114,826, issued on Feb. 8, 2011 all to Hermanson et. al. disclose preparing concentrated, readily pumpable soap based formulations containing greater than 40% of fatty acid(s).
U.S. Pat. No. 7,879,781 issued on February, 2011 to Patel et al. discloses preparing high emollient lamellar compositions consisting of 2-4% lauric acid resistant to viscosity changes under freeze-thaw cycles.
The rheological behavior of all surfactant solutions, including liquid cleansing solutions, is strongly dependent on the microstructure, i.e., the shape and concentration of micelles or other self-assembled structures in solution.
When there is sufficient surfactant to form micelles (concentrations above the critical micelle concentration or CMC), for example, spherical, cylindrical (rod-like) or discoidal micelles may form. As surfactant concentration increases, ordered liquid crystalline phases such as lamellar phase, hexagonal phase or cubic phase may form. The lamellar phase, for example, consists of alternating surfactant bilayers and water layers. These layers are not generally flat but fold to form submicron spherical onion like structures called vesicles or liposomes. The hexagonal phase, on the other hand, consists of long cylindrical micelles arranged in a hexagonal lattice. In general, the microstructure of most personal care products consist of either spherical micelles; rod micelles; or a lamellar dispersion.
As noted above, micelles may be spherical or rod-like. Formulations having spherical micelles tend to have a low viscosity and exhibit Newtonian shear behavior (i.e., viscosity stays constant as a function of shear rate; thus, if easy pouring of product is desired, the solution is less viscous and, as a consequence, it doesn't suspend as well). In these systems, the viscosity increases linearly with surfactant concentration.
Rod-micellar solutions are more viscous because movement of the longer micelles is restricted. At a critical shear rate, the micelles align and the solution becomes shear thinning. Addition of salts increases the size of the rod-micelles thereof increasing zero shear viscosity (i.e., viscosity when sitting in bottle) which helps suspend particles but also increases critical shear rate (point at which product becomes shear thinning; higher critical shear rates means product is more difficult to pour).
Lamellar dispersions differ from both spherical and rod-like micelles because they can have high zero shear viscosity (because of the close packed arrangement of constituent lamellar droplets), yet these solutions are very shear thinning (readily dispense on pouring). That is, the solutions can become thinner than rod micellar solutions at moderate shear rates.
In formulating liquid cleansing compositions, therefore, there is the choice of using rod-micellar solutions (whose zero shear viscosity, e.g., suspending ability, is not very good and/or are not very shear thinning); or lamellar dispersions (with higher zero shear viscosity, e.g. better suspending, and yet are very shear thinning). Such lamellar compositions are characterized by high zero shear viscosity (good for suspending and/or structuring) while simultaneously being very shear thinning such that they readily dispense in pouring. Such compositions possess a “heaping”, lotion-like appearance which convey signals of enhanced moisturization.
To form such lamellar compositions, however, some compromises have to be made. First, generally higher amounts of surfactant are required to form the lamellar phase. Thus, it is often needed to add auxiliary surfactants and/or salts which are neither desirable nor needed. Second, only certain surfactants will form this phase and, therefore, the choice of surfactants is restricted.
In short, lamellar compositions are generally more desirable (especially for suspending emollient and for providing consumer aesthetics), but more expensive in that they generally require more surfactant and are more restricted in the range of surfactants that can be used.
When rod-micellar solutions are used, they also often require the use of external structurants to enhance viscosity and to suspend particles (again, because they have lower zero shear viscosity than lamellar phase solutions). For this, carbomers and clays are often used. At higher shear rates (as in product dispensing, application of product to body, or rubbing with hands), since the rod-micellar solutions are less shear thinning, the viscosity of the solution stays high and the product can be stringy and thick. Lamellar dispersion based products, having higher zero shear viscosity, can more readily suspend emollients and are typically more creamy. Again, however, they are generally more expensive to make (e.g., they are restricted as to which surfactants can be used and often require greater concentration of surfactants).
In general, lamellar phase compositions are easy to identify by their characteristic focal conic shape and oily streak texture while hexagonal phase exhibits angular fan-like texture. In contrast, micellar phases are optically isotropic.
It should be understood that lamellar phases may be formed in a wide variety of surfactant systems using a wide variety of lamellar phase “inducers” as described, for example, in U.S. Pat. No. 5,952,286 titled “Liquid Cleansing Composition Comprising Soluble, Lamellar Phase Inducing Structurant” by Sudhakar Puvvada, et al., issued Sep. 14, 1999. Generally, the transition from micellar to lamellar phase are functions of effective average area of headgroup of the surfactant, the length of the extended tail, and the volume of tail. Using branched surfactants or surfactants with smaller headgroups or bulky tails are also effective ways of inducing transitions from rod micellar to lamellar.
One way of characterizing lamellar dispersions include measuring viscosity at low shear rate (using for example a Stress Rheometer) when additional inducer (e.g., oleic acid or isostearic acid) is used. At higher amounts of inducer, the low shear viscosity will significantly increase.
Another way of measuring lamellar dispersions is using freeze fracture electron microscopy. Micrographs generally will show lamellar microstructure and close packed organization of the lamellar droplets (generally in size range of about 2 microns).
One problem with certain lamellar phase compositions is that they tend to lose their lamellar stability in colder temperatures (e.g., 0 to 45 degree F.). While not wishing to be bound by theory, this may be because, in cold conditions, the oil droplets become less flexible and the spherical structure characterizing the lamellar interaction breaks into lamellar sheets instead.
Specific inventive concentrated lamellar cleansing compositions were unexpectedly discovered that provide consumers with (a) better in-use sensory properties such as creamy appearing lather and improved skin feel such as providing a moisturized feeling to the skin (b) better performance such as increasing the amount of lather and the speed to lather and (c) potentially offering more number of washes with a smaller pack/product for the user. The invention provides a concentrated cleansing composition characterized by a soft lamellar gel with a buffer-like feel and good spreadability with high levels of fatty acid(s) in a specific ratio range to synthetic anionic surfactants and cosurfactants. In a preferred embodiment, the inventive composition was unexpectedly found to be stabilized against a pronounced loss of viscosity after freezing and thawing by a selection of small hydrophobic molecules.
Concentrated cleansing compositions are known to go through undesired, very viscous phases (cubic and hexagonal phases) before the more desired, easy to handle, less viscous lamellar phase is reached upon increase of surfactant concentration (see e.g. U.S. 2007/0287648 to Moaddel et al, incorporated herein by reference). Co surfactants (herein defined as amphoteric or non-ionic surfactants or blends thereof) may be used to facilitate the formation of the lamellar phases. In the inventive concentrated formulations, a specific ratio range of synthetic anionic surfactant(s) and co-surfactant(s) to C12 to C18 fatty acid(s) was seen to allow the formation of lamellar phase and unexpectedly provide substantially improved lather volume and speed to lather. At these high levels of fatty acid content, though a soft lamellar gel is formed that is stable at room and high temperatures, an unusually large drop in viscosity (>95%) under freeze-thaw conditions was noted. In a preferred embodiment, it was unexpectedly found that the addition of specific, small, hydrophobic molecules substantially improved the freeze-thaw stability of the concentrated high fatty acid containing compositions that is essential in beneficial consumer use of the product.